Gold ores are treated by a variety of processes. All hydrometallurgical gold recovery processes rely on leaching relatively low concentrations of gold from ores using lixiviants, primarily cyanide solutions. Ores that contain gold extracted by comminuting and followed by leaching with cyanide solution are known as “oxide” or “free milling” ores. However, gold recovery from most ores by cyanide leaching is not effective, with as little as 30 percent, or even lower, of the gold content of the ore being amenable to cyanide leaching. These ores are commonly termed “refractory” ores. Poor gold recovery from refractory ores is typically caused by the gold being occluded in sulfide mineral grains (usually arsenopyrite and/or pyrite grains) so that the gold cannot react with the cyanide leach solution or by dissolved gold being adsorbed by carbonaceous material present in the ore (this phenomenon is known as “preg robbing”) Ores having both types of metallurgical problems (occluded and preg robbing) are commonly known as “double refractory” ores. Further losses in gold recovery can occur when dissolved gold is occluded by inorganic precipitates which typically occur during autoclave treatment of refractory gold ores.
A common method of treating refractory gold ores is by pressure oxidation in autoclaves. Pressure oxidation oxidizes sulfide minerals, rendering the residue non-refractory. The gold is then dissolved by cyanidation and concentrated by adsorption onto activated carbon (either in adsorption columns or in carbon added to the leaching process (known as Carbon-In-Leach (“CIL”) or Carbon-In-Pulp (“CIP”) techniques) or onto a resin (known as the Resin-In-Pulp (“RIP”) technique). The adsorbed gold is eluted from the adsorbed carbon by washing and stripping with ammonia, nitric acid, hydrochloric acid, caustic solution, and/or steam. The gold is then converted to a solid from the eluate by electrowinning (electroplating of gold onto cathodes), precipitation and filtration, or cementation.
The mining industry, particularly the gold mining industry, has been recognized as a source of mercury emissions to the atmosphere. Precious metal-containing ores, such as gold ores, commonly contain mercury. Mercury is often present in gold ores as a free mineral such as mercury sulfide (cinnabar or HgS), mercury-telluride minerals (eg. coloradoite), as minor constituent in sulfide minerals, as elemental mercury, an impurity in other sulfide minerals, and/or an amalgam with the elemental gold. A typical mercury concentration in such ores is from about 0.1 to about 200 ppm.
The strongly oxidizing conditions in the autoclave promote oxidation of sulfide sulfur to sulfate sulfur and dissolution of mercury species. For cinnabar, which contains most of the mercury in the ore, the reaction is:HgS+2O2→Hg2+SO42−  (1)
Because the conditions that promote the oxidation of gold bearing sulfides can also oxidize mercury associated with sulfides, it is possible to dissolve the majority of mercury in the autoclave slurry. The slurry, when discharged from the autoclave, is transferred to a flash chamber, where pressure is relieved, and a substantial amount of the liquid phase is converted to steam. The steam and off-gases can include a significant portion of the dissolved mercury, with the rest remaining in the aqueous and solid phases of the slurry.
The oxidized mercury can be difficult and expensive to remove from the steam and liquid phase of the slurry and, once recovered, to dispose of.